1. Field of the Invention
The disposal of waste has become an increasing problem in recent years and the need to convert many forms of waste into energy has become apparent. However, this must be done in an environmentally acceptable manner and at reasonable cost.
2. Description of Related Art
There have been many attempts to convert waste into energy. The most commonly used to date has been incineration. However, this is becoming increasingly unacceptable on account of emissions and public reaction.
Potentially the most acceptable means of conversion of waste into energy is that by pyrolysis and gasification leading to the production of a fuel gas which is preferably oxidised whilst it is still hot to release the energy. Alternatively, the pyrogas may be condensed to yield a complex hydrocarbon oil and gas which may be burnt. However this is difficult within environmental constraints and the immediate combustion of the fuel gas is preferred.
Many of the pyrolysis and gasification systems which are used at present, or which have been proposed for use, employ rotational or moving conveyors or containers to feed, mix and agitate the waste material which is to be pyrolysed. However, there are many problems, particularly relating to the feeding and sealing of these rotational systems, and one of the purposes of the present invention is to solve these problems in an economic manner and at the same time provide an efficient continuous pyrolysis and gasification process which can be closely controlled. Additionally, the present invention enables effective feeding, mixing and agitation of the waste so that it is subjected to the pyrolysis and gasification process in a uniform manner.
In particular, the invention permits a wide range of waste materials to be continuously pyrolysed and gasified using the thermal reaction to heat the process in an integrated compact installation. The waste heat which is recovered can be used for power generation and the thermal drying of the waste.
In our co-pending U.K. Patent Application No. 9521984.6 there is described a process for the production of heat energy from solid carbonaceous fuels which process comprises subjecting the carbonaceous fuel to substantially anaerobic pyrolysis in at least one first zone and thereafter transferring the char resulting from the pyrolysis to a second zone which is segregated from the first zone or zones, the char being subjected to gasification in the second zone by introduction of primary combustion air, optionally with steam, the off gases from the second zone and the pyrolysis gases from the first zone or zones being thereafter subjected to secondary combustion and the first zone or zones being heated by heat derived from said secondary combustion, ash being removed from the bottom of the second zone. The present invention has particular applicability to such a process and apparatus, and provides a particularly efficient means of feeding carbonaceous fuel materials to the pyrolysis stage of such a process.
As previously mentioned there are problems in providing effective sealing of pyrolysis and gasification apparatus, particularly when involving moving parts, and particularly in the part of the apparatus where the carbonaceous fuel is introduced. Several kinds of mechanical charging arrangements are known involving various kinds of gates, air locks and ramming means. However, the moving parts of such mechanical arrangements tend to be complicated, are prone to both inward and outward gaseous leakage and also to jamming due to accumulation of the carbonaceous material and the pyrolysis products thereof. Moreover, the preferred form of pyrolysis reactor is a tube because this provides one of the most effective means of transferring heat to the material to be pyrolysed. However, in feeding systems involving the forcing of a particulate material into or through a tubular vessel it is found that due to the effect of friction, which is exacerbated for example if the tube is constricted, for example due to accidental damage to the interior of the tube, interior surface irregularity or intentional restrictions, for example in the form of devices for improving mixing and/or heat transfer, the fed material tends to jam in the tube, for example at such a restriction, and sometimes the application of a very significantly higher force than that needed to cause normal feeding into and through the tube may not even be sufficient to overcome the jamming. Moreover, such situations are, of course, wasteful of energy and are uneconomic in continuous production terms due to the interruptions in normal running which are caused by the jamming.
The present invention provides solutions to the above-mentioned problems which are inherent in the use of such reactors.
GB-A-1 057 977 discloses a method of feeding dry dust-like solid fuels to a high pressure gasification system by adding a volatile liquid hydrocarbon to the feed to make a semi-solid mixture which is pumpable.
DE-A-4 327 430 describes a device and procedure for the thermal disposal of waste in which compacted slugs of waste are initially passed into a heated degasification tube. However, no separate seals are provided in the tube and no reference is made to possible jamming of the slugs in the tube or how such jamming may be avoided.
GB-A-262 901 describes a process and apparatus for feeding solids into or removing solids from vessels under pressure involving forcing the solids through a tube by means of a piston or plunger, optionally including a gas-tight material in the tube at each stroke of the plunger. It is suggested to make the tube slightly conical in cross-section.